Nanoparticles are classified as fine particles sized between 1 and 100 nm. Significant research into the functions and uses of nanoparticles has been conducted since nanoparticles have been found to have utility in a broad range of fields, such as biomedicine, materials science, electronics, consumer products, pharmaceuticals, cosmetics, transportation and energy (see Hoeppner and Novobry 2012. Quarterly reviews of Biophys 45:209-255; and Wong 2012 Microchimia Action 117:245-270). Whilst nanoparticles formed from carbon material or dendrimers are known, much interest is also focused on metal nanoparticles or metallised nanoparticles. These are of particular interest for many applications in optoelectronics, electronics, biochemistry (including plant biochemistry) and medicine.
However, the production of metallic and metallised nanoparticles poses a number of problems. Metallic nanoparticles are commonly synthesized using hazardous chemicals such as sodium borohydride, tetrakishydroxymethylphosphonium chloride (THPC), poly-N-vinyl pyrrolidone (PVP), and hydroxylamine (Narayanan and Sakthivel, 2011 Adv. Colloid Interface Sci. 169:59-79). Other methods of nanoparticle production use aerosol deposition, laser ablation and lithography; which are not considered environmentally friendly. Moreover, the synthesis of metallic nanoparticles is expensive and time-consuming (Narayanan and Sakthivel, 2011, supra). Therefore, there is a clear need for simple, cost-effective and eco-friendly methods for the production of metallic nanoparticles (Raveendran et al., 2003 J. Am. Chem. Soc. 125:13940; Sharma et al., 2009 J. Colloid Interface Sci. 145:83; Narayanan et al., 2011 J. Colloid Interface Sci. 156:1). In recent years, it has been shown that biological systems including plants, algae (Govindaraju et al., 2008 J. Mater. Sci. 43:5115), diatoms (Scarano et al., 2002 Biometals 15:145; Scarano et al., 2003 Plant Sci 165:803), bacteria (Lengke et al., 2007 Langmuir 23:2694-2699) and human cells (Anshup et al., 2005 Langmuir 21:11562) can convert inorganic metal ions to metal nanoparticles. This typically uses a reductive process facilitated by proteins and metabolites contained in these organisms. Plant-based nanoparticle production has significant advantages over other biological systems. For example, plants are low maintenance, do not require sterile growth conditions and rapidly produce biomass. This is in contrast to the high cost and maintenance required to culture mammalian and bacterial cells. The low cost of cultivation, short time frame for production, safety and ability to scale up or down depending on demands make plants a particularly attractive nanoparticle synthesis platform.
Plant sap extracts can readily be used to reduce metal ions into metallic nanoparticles. This approach involves less downstream processing in order to extract the synthesized particles than other biological systems. The feasibility of this process has been demonstrated extensively using sap from a plethora of different plant species and a variety of metal acids and salts (including copper, gold, silver and iron). For example, Pelargonium graveolens extracts reduced gold ions into 20-40 nm decahedral and icosahedral shaped nanoparticles and stabilized them (Shankar et al., 2003, J. Mater Chem 13:1822), whereas Cymbopogon flexuosus extracts produced gold spheres and nanotriangles, ranging in size from 0.05-18 μm (Shankar et al., 2004 Nat Mater 3:482). Azadirachta indica extract has been used to reduce chloroauric acid (HAuCl4) into gold planar triangles and hexagons ranging in size from 50-100 nm (Shankar et al., 2004 J. Colloid Interface Sci 275:496). This study also reported that A. indica sap can reduce silver nitrate into polydispersed spherical nanoparticles of 5-25 nm in size (Shankar et al., 2004 J. Colloid Interface Sci 275:496). In addition, Aloe barbadensis leaf extract has been used to produce cubic 5-50 nm In2O3 particles (Maensiri et al., 2008 J Optoelectron Adv Mater 10:161). The nanoparticles formed are predominately metal in content.
However, it should also be noted that in addition to dispersed nanoparticles these techniques often produce massive micron-sized metallised and/or metallic aggregations significantly lowering the yield of “real” nanoparticles (see FIGS. 1a, 2 and 3). In the context of the present invention, as understood by those in the present field, a “real” nanoparticle, as discussed herein, is considered to be a metallised and/or metallic nanoparticle that has at least one dimension less than 100 nm. Moreover, these processes do not always allow preparation of uniform (in size and architecture) nanoparticles with a specific morphology, which is highly important for many technological applications.
In a separate approach, viruses or virus-like particles were shown to serve as biotemplates for metal deposition, forming metallised nanoparticles in a specific manner (Kadri et al., 2011 Virus Research 157:35-46; Aljabali et al., 2010 Nanoscale 2:2596-2600). For example, the external or internal surfaces of cowpea mosaic virus (CPMV), brome mosaic virus (BMV), potato virus X (PVX) and tobacco mosaic virus (TMV) particles have been coated with different metals. Since TMV particles expose several distinct surfaces suitable for metallization (Balci et al., 2012 Nanotechnology 23; Knez et al., 2006 Nano Lett. 6: 1172-1177) or functionalization with organic molecules (Endo et al., 2006 Chemistry 12:3735-3740; Shimizu et al., 2005 Chem Rev 105:1401-1443), they have been exploited as promising biotemplates for nanoscale materials (Lee et al., 2012 Biotechnol. Bioeng. 109:16-30). Due to their physical and chemical stability they are attractive tools for inorganic nanostructures (Bitner et al., 2005 Naturwissenschaften 92:51-64, Dujardin et al., 2003 Nano Lett. 3:413-417). Dujardin et al. (2003, supra) and Knez et al. (2006, supra) used the plant-derived virions to produce nanowires by selective metallization of the outer surface or the inner channel. For fabrication of thin film-embedded nanostructures, TMV was utilized as a nanobiotemplate in atomic layer deposition systems producing metal oxide nanotubes (Knez et al., 2006, supra). Most importantly, functional electronic devices (Atanasova et al., 2011 Adv. Mater. 23:4918-4922; Chen et al., 2011 Electrochim. Acta 56:5210-5213) as well as ferrofluids (Wu et al., 2010 ACS Nano 4:4531-8) were designed on the basis of TMV biotemplates.